Classic Sudoku
Classic Sudoku is played on a 9x9 grid divided into nine 3x3 boxes. The goal is to fill every empty cell with a digit from 1 to 9.
Rules
- Each row must contain every digit from 1 to 9 exactly once.
- Each column must contain every digit from 1 to 9 exactly once.
- Each 3x3 box must contain every digit from 1 to 9 exactly once.
- Digits already shown in the grid are givens and cannot be changed.
- A valid puzzle has only one solution.
Notation used in this help
R3C4means row 3, column 4.- A house is a row, column or 3x3 box.
- A candidate (pencil mark) is a digit that can still go in an empty cell.
Solving strategies
Use the following classic Sudoku techniques from the simplest to the most advanced. The hint button and Solving Steps can help you find the next step.
Singles
| Technique | What it does |
|---|---|
| Full House | A row, column or box has only one empty cell; fill it with the missing digit. |
| One Digit Left | The grid already contains eight copies of a digit; place the last copy in the only cell that can take it. |
| Naked Single | A cell has only one candidate left; place that digit. |
| Hidden Single | A digit has only one possible cell in a row, column or box; place it there. |
Candidate elimination
| Technique | What it does |
|---|---|
| Classic candidates removal | When a digit is placed, remove it from the row, column and box of that cell. |
| Naked Pair / Triple / Quad | Two, three or four cells in one house contain only the same two, three or four candidates. Remove those candidates from the rest of the house. |
| Hidden Pair / Triple / Quad | Two, three or four digits can only appear in the same two, three or four cells of a house. Remove other candidates from those cells. |
| Pointing Set | A candidate in a box is restricted to one row or column. Remove it from the rest of that row or column. |
| Box Line Reduction | A candidate in a row or column is restricted to one box. Remove it from the rest of that box. |
| X-Wing | A candidate forms a rectangle in two rows and two columns. Remove it from the other cells in those rows or columns. |
You can also apply the same techniques to variant houses, such as diagonal houses or Windoku windows, when the grid contains those constraints. See the Variants page for variant-specific rules and solving tips.
Detailed technique guides
The techniques above are explained with example grids on separate pages:
- Singles - Full House, One Digit Left, Naked Single and Hidden Single.
- Candidate removal - removing a placed digit from its row, column and box.
- Pairs and triples - Naked and Hidden Pairs and Triples.
- Quads - Naked and Hidden Quads.
- Pointing and claiming - Pointing Set and Box Line Reduction.
- X-Wing - the X-Wing technique.
Hints and Solving Steps
- When you are stuck on the Play page, use the hint button to see the next solving step and the technique behind it. See Play help.
- When you are constructing a grid on the Set page, use the Solving Steps button to see a complete solving path. See Set help.
- Each step names the technique you can use and highlights the cells or candidates involved.
- For classic Sudoku, every hint is based on logic and no guessing is required. If you cannot make progress with these techniques, the hint dialog reports that no hint is available.
Difficulty and Length
Grids are rated by difficulty and length. Difficulty depends on the hardest technique you need to solve the grid, and length is based on the solving steps and their complexity (tediousness). You can filter grids by both on the home page. See Filters.
Try it
- Set (construct) a Sudoku - build your own grid.
- Keyboard Shortcuts - speed up entering digits.
- Settings - customize play and set/solve behavior.